In an image forming apparatus that forms an image electrophotographically, a developing roller is provided opposite to a photosensitive drum. An electrostatic latent image is formed on a surface of the photosensitive drum. The developing housing accommodates toner. The developing roller is rotatably held in the developing housing. A developing bias is applied to the developing roller. When the photosensitive drum is rotated and an electrostatic latent image on the surface of a photosensitive drum opposes the developing roller, toner is supplied from the developing roller to the electrostatic latent image by means of a potential difference between the photosensitive drum and the developing roller. According thereto, the electrostatic latent image is developed into a toner image, and the toner image is formed on the photosensitive drum. The toner image is transferred to a sheet directly or by way of an intermediate transfer belt from the surface of the photosensitive drum.
However, when the toner image is formed in the number of times, the toner held on the developing roller may be deteriorated. For example, quantity of electric charge of the toner may be decreased. Thus, the quantity of toner transferred from the developing roller to the surface of the photosensitive drum may eventually vary, and density of a toner image formed on a sheet differs from an appropriate density. Therefore, the image forming apparatus corrects a value of the developing bias at appropriate timing in order to form a toner image at an appropriate density.
In order to correct the developing bias, a known correction processing utilizes a test toner image called a patch. In the known correction processing, for instance, a patch is experimentally formed on a surface of an intermediate transfer belt. Then, the density of the patch is measured, and the value of the developing bias is corrected such that a deviation between the measured density and a predetermined target density comes to zero.